Someone is having an issue with the downloaded member list. ... TheyIt has been so long since I've run a Windows machine without Office installed that I'm not entirely sure of this, but it may be possible that their Windows system does not have a file association set up for the .csv extension. If Office were installed, files with that type would be associated with (automatically open in) Excel.
As Jim Ruby said, there ought to be a "Open With" button either in the download, or as a right-click on the downloaded file, which will allow the user to select Notepad or Wordpad. With that should be a checkbox to "Always use this program" for this type of file. Checking that (or leaving it checked) should set up the file association for future handling of .csv files.
Or, failing that, if you're trying to open the downloaded .csv from within Notepad you need to change the file type filter to *.* so that it will show you the .csv (and all other files).
Then there is the separate issue of extracting just the info you need from the file. I'll agree with those who've said that using a spreadsheet program is the easiest way, if you have such a program (and I can't imagine life without one).
Once you've got the file open in Notepad, the first problem with Groups.io's export is that it uses the unix-style line termination: just a LineFeed (U+000A). Notepad doesn't recognize those as new lines, so the rows of the export get mashed together. This should be taken as a bug report (or feature request) - the export should use (or offer) ASCII standard line termination - aka CRLF (U+000D, U+OO0A) for broadest interoperability.
Fortunately Groups.io's export has the email address as the first column, and the display name as the second. Which (had it not been for the line termination issue) would have made it relatively easy for the Notepad user to go down from line to line, deleting the unneeded information from the end of each line.